Re: [ROOT] error calc. in TProfile

From: Rene Brun (
Date: Wed Jul 17 2002 - 13:07:31 MEST

Hi Martin,

I agree with your theory. In the practical case of TProfile
with low statistics in one bin and very small weights, do you have a
better algorithm to propose? I mean an algorithm really working,
and for example, not subject to rounding problems?
Let me know. I will be happy to include it.

Rene Brun

On Wed, 17 Jul 2002, Martin Kestel wrote:

> Hi Rene,
> RB>The computation of errors in TProfile has evolved with time
> RB>in case of bins with low statistics (1, 2 or 3 entries).
> RB>In version 3.02/07, we introduced a new condition, in case
> RB>the errors computed are very small (error/content <1e-6)
> RB>We had complaints from several users making fits on such profiles
> RB>and finding that too much weight was given to the points
> RB>with low statistics.
> RB>Any idea to improve the existing algorithm is welcome.
> as an old PAW user, I know that the error calculation within PAW has always
> been the source of major trouble.
> Now, errors calculated for histograms have a meaning (usually). Making fits, I
> need to be sure that the errors calculated for histograms are representative
> of the data. If in a certain bin in profile histograms there are only few
> entries, the error is going to be large (usually), independent of spread
> option or error-on-the-mean option.
> Now, when the error is large, the point will get a low weight in a fit, that's
> just how fits are set up to function. Therefore I can hardly understand how it
> can happen that such data points get large weights.
> Introducing a fix or a fudge every time someone complains does not seem to be
> a good policy; in the end we (as users) want to make serious physics with ROOT
> and need to be able to rely on such basic things like error calculation.
> I do not see, why the spread option and the error-on-the-mean options shold
> not suffice. They are meaningful and very (really!!!!) useful.
>   just my .02$
>     Martin

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